The invention is directed to a single mode optical waveguide fiber having controlled negative total dispersion and a relatively large effective area. In particular, the single mode waveguide has a total dispersion which is less thanxe2x80x94100 ps/nm-km.
Several factors have combined to make the wavelength range, 1500 nm to 1600 nm, most preferred for telecommunication systems incorporating optical waveguide fiber. These are:
the availability of reliable lasers in the wavelength window around 1550 nm;
the invention of the optical fiber amplifier having an optimum gain curve in the wavelength range 1530 nm to 1570 nm;
the availability of systems capable of wavelength division multiplexing of signals in this wavelength range; and,
the availability of waveguide fibers having a low dispersion to compliment the very low attenuation over this wavelength range.
These advances in technology make possible very high information rate, multi-channel telecommunication systems which have a large spacing between stations where the signal is electronically regenerated.
However, many telecommunication systems installations pre-date the technological advances which make 1550 nm the most preferred operating window. These earlier systems were designed primarily for use over a wavelength range centered near 1310 nm. The design includes lasers which operate at wavelengths near 1310 nm and optical waveguides which have a zero dispersion wavelength near 1310 nm. The waveguide fiber, in these systems, does have a local attenuation minimum near 1310 nm, but the theoretical minimum at 1550 nm is about half that at 1310 nm.
A strategy has been developed to make these older systems compatible with the new laser, amplifier, and multiplex technology. As disclosed in U.S. Pat. No. 5,361,319, Antos et al., (""319, Antos) and discussed further in the references noted therein, an essential feature of this strategy is to overcome the relatively high total dispersion by inserting into each waveguide fiber link a length of waveguide fiber which compensates for the total dispersion of the link at 1550 nm. The term xe2x80x9clinkxe2x80x9d used herein is defined as the length of waveguide fiber which spans the distance between a signal source, i.e., a transmitter or an electronic signal regenerator, and a receiver or another electronic signal regenerator.
The ""319, Antos patent recites a dispersion compensating waveguide fiber having a core refractive index profile which provides a dispersion at 1550 nm of about xe2x88x9220 ps/nm-km. The dispersion sign convention common in the art is that a waveguide dispersion is said to be positive if shorter wavelength light has a higher speed in the waveguide. Because the dispersion at about 1550 nm of a waveguide fiber, having a zero dispersion wavelength near 1310 nm, is about 15 ps/nm-km, the length of dispersion compensating waveguide fiber required to fully compensate for total dispersion at 1550 nm is 0.75 of the original link length. Thus, for example, a 50 km link of waveguide fiber has a total dispersion at 1550 nm of 15 ps/nm-kmxc3x9750 km=750 ps/nm. To effectively cancel this dispersion, a length of dispersion compensating waveguide fiber of 750 ps/nm÷20 ps/nm-km=32.5 km is required.
The additional attenuation introduced into the link by the dispersion compensating waveguide would have to be offset by means of an optical amplifier. The introduction of additional electronic regenerators into the link would not be cost effective. Further, the cost of the dispersion compensating waveguide fiber is a significant fraction of the total waveguide fiber cost. The long lengths of dispersion compensating waveguide required must be formed into an environmentally stable package which can take up considerable space.
Because the compensating waveguide fiber design usually has more refractive index modifying dopant in the core region, the attenuation is, in general, higher relative to the standard waveguide fiber in a link.
The higher signal power level, made possible by improved lasers and by optical amplifiers, as well as wavelength division multiplexing, increases the possibility that link length or data transmission rate may be limited by non-linear optical effects. The impact of these non-linear effects can be limited by increasing the effective area (Aeff) of the fiber. The effective area is Aeff =2xcfx80(∫E2r dr)2(∫E4r dr), where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light. The distortion due to non-linear effects depends upon a term of the form, Pxn2/Aeff, where P is the signal power, and, n2 is the non-linear index constant. Thus, in the design of a dispersion compensating waveguide fiber, care must be taken to insure that Aeff of the compensating fiber is large enough so that the compensation fiber does not cause significant non-linear effects in the link. If Aeff of the compensating fiber is smaller than that of the original fiber in the link, the compensating fiber may be placed at a link location where signal power is lower and thus non-linear effects minimum. Also, in many links the smaller Aeff compensating fiber is a small fraction of the overall link length and so does not contribute significantly to non-linear distortion of the signal.
Thus, there is a need for a dispersion compensating optical waveguide fiber:
having a length which is a small fraction, e.g., less than 15%, of the link length;
which is sufficiently low in attenuation to eliminate the need for additional signal amplification solely to offset the compensating waveguide fiber attenuation; and,
which has an effective area sufficiently large to preclude non-linear dispersive effects in the compensating waveguide fiber from being a limiting factor.
The effective area is
Aeff=2xcfx80(∫E2r dr)2/(∫E4r dr),where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light.
The non-linear discriminator factor is defined by the equation
Gnl=n2/Aeff(exp[D1xc3x97L1/Dd/xcex1]xe2x88x921)/xcex1, where n2 is the non-linear refraction coefficient, D1 is the dispersion of the portion of the waveguide optimized for operation around 1310 nm, L1 is the length corresponding to D1, Dd is the dispersion of the compensating waveguide fiber and a xcex1 the attenuation of the dispersion compensating fiber. This expression for Gnl derives from a base definition Gnlxcx9cn2/Aeff(Effective lengthxc3x97Output Power). The effective length and output power are expressed in terms of waveguide fiber length and attenuation, xcex1. The compensating waveguide fiber is introduced into the equation via the requirement D1xc3x97L1=Ddxc3x97Ld. Gnl is a useful quantity in evaluating the efficiency of a link because it is a combination of system factors such as system architecture, amplifier spacing, Dd/xcex1, and, n2/Aeff.
The invention disclosed herein meets the requirements for an improved dispersion compensating waveguide fiber. A species of the genus of segmented core refractive index profiles, introduced in U.S. Pat. No. 4,715,679, Bhagavatula and in U.S. patent application Ser. No. 08/378,780, Liu, has been discovered which are uniquely suited for dispersion compensating waveguide fiber.
A first aspect of the invention is a single mode optical waveguide fiber having a central core glass region and a surrounding layer of clad glass. The core glass region has at least three segments, each of which is characterized by a refractive index profile, a radius, r, and a xcex94%. The definition of the % index delta is % xcex94=[(n12xe2x88x92nc2)/2n12]xc3x97100, where n1 is a core index and nc is the clad index. Unless otherwise stated, n1 is the maximum refractive index in the core region characterized by a % xcex94. The radius of each segment is measured from the centerline of the waveguide fiber to the point of the segment farthest from the centerline. The refractive index profile of a segment gives the refractive index value at the radial points of that segment. In this first aspect of the invention, xcex941 %, the delta percent of the first segment, is positive and the xcex94 % of at least one other segment is negative. The radii and xcex94 %""s of the segments are chosen to provide a negative total dispersion at 1550 nm which is no greater than xe2x88x92150 ps/nm-km.
In an embodiment of this first aspect, the core glass region has three segments and the second segment has a negative xcex94 %. A preferred embodiment has respective segments, beginning at the first segment and proceeding outwardly, having radii in the ranges of about 1 to 1.5 xcexcm, 5.5 to 6.5 xcexcm, and, 8 to 9.5 xcexcm, and, the respective segments, beginning at the first segment and proceeding outwardly, having xcex94 %""s in the ranges of about 1.5 to 2%, xe2x88x920.2 to xe2x88x920.5%, and, 0.2 to 0.5 to provide an effective area, Aeff, at 1550 nm, no less than about 30 xcexcm2. Effective areas higher than 60 xcexcm2 are achievable.
In another embodiment of this first aspect, the core glass region has four segments, and the second and fourth segments have a negative xcex94 %. A preferred embodiment has respective radii, beginning at the waveguide center and proceeding outward, in the ranges of about 1 to 2 xcexcm, 6 to 8 xcexcm, 9 to 11 xcexcm, and, 13 to 17 xcexcm. The corresponding segment xcex94 %""s are in the respective ranges of about 1 to 2%, xe2x88x920.2 to xe2x88x920.8%, 0.4 to 0.6%, and xe2x88x920.2 to xe2x88x920.8%. These preferred core profiles provide Aeff at 1550 nm of no less than 30 xcexcm2. The dispersion slope of 2 to 15 ps/nm-km provided by these core profiles is reasonably small.
In another embodiment of this aspect of the invention, the core glass region has four segments, numbered 1 to 4, beginning at the waveguide fiber center. The corresponding relative refractive index percent of the segments are ordered as xcex941 % greater than xcex943 % greater than xcex944 % greater than xcex942 %, where xcex942 % is negative. The respective xcex94 %""s are, 1.5 to 2% for xcex941 %, xe2x88x920.2 to xe2x88x920.45% for xcex942 %, 0.25 to 0.45 % for xcex943 %, and, 0.05 to 0.25% for xcex944 %, and the respective radii associated with these xcex94%""s are in the ranges of about 0.75 to 1.5 xcexcm for r1, 4.5 to 5.5 xcexcm for r2, 7 to 8 xcexcm for r3, and, 9 to 12 xcexcm. In this embodiment, the total dispersion slope is negative which serves to cancel with the positive slope of the waveguide fiber of the original link operating in the 1310 nm window. Typically, the negative slope of the total dispersion is in the range of about xe2x88x920.1 to xe2x88x925.0 ps/nm2-km.
A second aspect of the invention is a single mode optical waveguide fiber link made of a first length of single mode fiber designed for operation in the 1310 nm window and a length of dispersion compensating single mode waveguide fiber. The dispersion compensating fiber length and total dispersion product at 1550 nm are chosen to add algebraically with the length times dispersion product of the first length of waveguide fiber to produce a pre-selected value of total dispersion for the link. The pre-selected value may advantageously be chosen zero at 1550 nm to provide the lowest total dispersion over this window. If four wave mixing or self phase modulation is an anticipated problem for 1550 nm window operation, the total dispersion at 1550 nm may be selected to be a small positive number.
The attenuation of the dispersion compensating waveguide fiber is held to a low value so that attenuation does not become a data rate limiter for the link. In addition, Aeff should be large enough, at least 30 xcexcm2, so that significant non-linear dispersive effects are not introduced by the dispersion compensating waveguide fiber. The ratio of the compensating fiber total dispersion and attenuation, together with Aeff are combined in a function which describes a discriminating factor, denoted Gnl in the art and defined above, which is a measure of the properties of the compensating waveguide fiber with regard to non-linear dispersive effects.
An embodiment of this aspect of the invention includes a dispersion compensating waveguide fiber which has a total dispersion, Dd no greater than xe2x88x92150 ps/nm-km, Aeffxe2x89xa730 xcexcm2, and the magnitude of Dd/xcex1xe2x89xa7150 ps/nm-dB. In a preferred embodiment, the magnitude of Dd/xcex1 is xe2x89xa7250 ps/nm-dB.
Because the total dispersion of the compensating fiber is a large negative number, the length of compensating fiber required to arrive at a pre-selected value of total dispersion for the link is generally less than 15% of the link length and may be less than 5% of the link length.
A third aspect of the invention is a method of making a single mode optical waveguide which compensates at 1550 nm for dispersion in a link originally designed for operation in the 1310 nm window. The draw preform, comprising a central core glass region and a surrounding clad glass layer, the core glass region having the properties described in the first aspect of the invention, may be made by any of several techniques in the art. These include, inside and outside chemical vapor deposition, axial chemical vapor deposition and any of the modifications of these techniques in the art. The core regions having positive relative refractive index may be formed using a dopant such as germania in a silica glass matrix. The core regions of negative relative index may be formed using a dopant such as fluorine.
A drawing tension greater than about 100 grams has been found to yield better total dispersion to attenuation ratios than similar waveguide fibers drawn at lower tension. To limit loss due to bending, an outside diameter greater than about 125 xcexcm is preferred. The upper limit on outside diameter is set by practical limitations such as cost and required cable size. A practical upper limit is about 170 xcexcm.
To limit attenuation due to residual coating stress, the coated waveguide fiber may be loose wrapped on a spool and heat treated. For most effective stress relief, the spool size should be greater than about 45 cm. The winding tension used to wrap the waveguide fiber onto the spool is less than about 20 grams. A preferred winding method is one in which the waveguide fiber is allowed to assume a catenary configuration just prior to being wound onto the spool.
A heat treatment at a temperature at least 30xc2x0 C. greater than the glass transition temperature, Tg, of the polymer coating and continued for 1 to 10 hours has been found effective to relieve residual coating stresses for the coating types and thicknesses used in testing. A holding time of about 5 hours was found to be effective for the thickness, about 60 xcexcm, of UV cured acrylate coating used in the manufacture of the waveguide fiber described herein.
It is understood that the heat treating method recited herein includes temperature and time limitations suited to any of the several polymer coatings types and thicknesses suitable for use in the manufacture of optical waveguide fiber.